Such an agitator mill, as shown in DE 41 42 213 A1, has an outer, cylindrical grinding container, which is fixed and connected to a coaxially spaced, cylindrical inner stator. The grinding container and inner stator are closed at their lower end by means of a circular base, so that between the outer grinding container and the inner stator is formed a circular grinding area. A cover is also mounted on the upper end of the grinding container.
A downwardly open cup-shaped rotor is mounted in rotary manner coaxially to the grinding container and inner stator and comprises a rotor base positioned in the vicinity of the grinding container cover and a downwardly directed cylindrical part connected thereto. The cylindrical part extends into the grinding area formed between the grinding container and the inner stator. Thus, the grinding area is subdivided into a cylindrical outer grinding area and a cylindrical inner grinding area coaxial thereto, the two grinding areas being hydraulically connected by means of a lower connecting area at the lower end of the cylindrical part of the rotor.
Into the inner stator is integrated a discharge device, which has a discharge tube issuing into the upper region of the inner stator. The upper mouth or opening of the discharge tube is surrounded by a cylindrical filter or separating screen, which is closed at the top and has over its circumference a screening surface.
In order to sufficiently comminute the mill charge, on the grinding container, inner stator and cylindrical part of the rotor are provided agitating bolts. Moreover, the inner and outer grinding areas contain grinding beads, which are in particular balls made from glass, minerals, steel or the like.
The mill charge is fed into the outer grinding area, normally under a slight pressure, in the form of a slurry, i.e. as dry material with internal liquid, in the upper region of the grinding container cover, so that it mixes with the grinding beads. The mill charge and grinding beads flow downwards through the outer grinding area, flows below the rotor in the lower connecting area and then rises upwards in the inner grinding area, where it passes on to the separating screen at the upper end. The separating screen retains the grinding beads and coarse mill charge fractions and they flow back via radially directed overflow ducts into the outer grinding area, whereas the fine fractions pass through the separating screen into the discharge device.
In earlier agitator mills the separating screen was fitted to the inner stator and was consequently also fixed. It has been found that such a construction leads to several disadvantages. Firstly there can be a short-circuit flow through the overflow ducts between the outer and inner grinding areas, so that the mill charge does not flow first through the outer and inner grinding areas, so that the grinding capacity of the agitator mill is significantly reduced. It has also been found that in the case of a high throughput and high mill charge viscosity, the separating screen becomes relatively rapidly clogged, which also leads to an undesired pressure build-up in the agitator mill.
To prevent the clogging of the separating screen an attempt has been made to provide on the inside of the rotor stripping edges extending to the separating screen surface and passing over the same. However, it is not possible to in this way achieve the desired separating screen cleaning action. It has in fact been found that the mill charge is smeared by the stripping edges and the separating screen clogs even more rapidly, even if the grinding beads are transported outwards.
From the aforementioned DE 41 42 213 A1 it is known to fit the separating screen to the discharge device located in the inner stator and to mount the same rotatably therewith in the inner stator, a separate motor being required as the drive. The rotation of the separating screen leads to a self-cleaning due to centrifugal force and within the mill charge builds up an outwardly directed pressure, so that an inwardly directed short-circuit flow through the overflow ducts is reliably avoided. However, the constructional effort and expenditure for the rotary bearing of the discharge device in the inner stator and the arrangement of a separate drive motor is very high. It is also necessary to connect the rotary discharge tube, outside the grinding container, to a further extending, fixed line network, which also requires a complicated transition construction.
The problem of the invention is to provide an agitator mill of the aforementioned type, in which a correct operation can be ensured at relatively low constructional costs.